Almost every railway system worldwide use trackside signals to ensure safety and maintain an orderly flow of traffic. In North America, signaling is typically implemented using green, red, and yellow electric lights a traditional signal can include a single light or multiple lights, which, depending on the given state of illumination, present a given aspect conveying a particular indication. (These signals can use individual incandescent bulbs and lenses in a single housing for each color, or use searchlight units, which change color using mechanical or electrical mechanisms in response to electrical control signals.)
There are a number of permissive and absolute indications that can be represented by the signal aspect. For example, in a signal employing two vertically aligned signal lights, an aspect with an illuminated green light above an illuminated green light is typically a “clear” indicating that the train crew can proceed along the upcoming block of track. In contrast, an aspect having an illuminated red light over another illuminated red typically indicates an “absolute stop” to the train crew. For a single light signal, an illuminated green light on a signal stanchion, with or without a numbered plate, is typically also a “clear” indicating that the train crew can proceed. On the other hand, a single illuminated red on a stanchion with a numbered plate typically indicates “stop and precede” at restricted speed, while a single illuminated red on a stanchion without a numbered plate typically indicates “absolute stop”.
Given the significant need to maintain safety by ensuring compliance with signal aspects, efficient and accurate techniques are necessary for monitoring train crew signal compliance, and particularly “red aspect” compliance.